Adhesion And Cohesion

Adhesion And Cohesion

Write a short note on adhesion and cohesion.
Answer:

1. Adhesion:

Adhesion is defined as a “state in which two surfaces are held together by interfacial forces which may consist of valence forces or interlocking forces or both “American Society for Testing and Materials”.

Concept of Adhesion in Dentistry:

Retention and stabilization of restoration require the removal of sound tooth structure at the time of restorative procedures, but in adhesive restorations, the destruction of intact enamel and dentin is not necessary since retention is mainly adhesive and does not depend on mechanical retentive forces. This results in a tight tooth–a restorative interface that prevents microleakage.

Criteria for Adhesion:

For proper adhesion, interacting surfaces must be attached to each other at their interface regardless of their phases. Factors governing the ability of the adhesive to make intimate contact with the substrate are:

• Characteristics of the substrate.
• Wettability of the substrate by an adhesive
• Viscosity of adhesive
•  Morphology or surface roughness of the substrate.

Clinical Applications of Adhesion:

The common aspects of clinical dentistry where the principles of adhesion are applicable are as follows:

• Adhesion of bacterial plaque biofilm to the enamel.
• Chemical, mechanical, and micromechanical retention of restorative materials to the tooth structure.
• Mechanical and micromechanical adhesion of fixed partial dentures.
• Retention of orthodontic brackets onto the enamel surface
• Retention of posts in endodontically treated teeth.
•  Adhesion between the impression tray and the elastomeric
  impression material with the help of tray adhesives.

Factors Affecting Adhesion:

Following are the factors which affect adhesion with the tooth structure, i.e…

• Composition: Enamel and dentin consist of both organic and inorganic components at different proportions. A material that adheres to the inorganic component may not adhere to organic components making adhesion
  difficult.
• Smear layer: As the tooth is prepared for receiving any restoration, the smear layer formed as a result of cutting interferes with proper wetting and also reduces the surface energy of the substrate, whether it is enamel, dentin, or
  cementum resulting in poor adhesion.
• Saliva: Another problem associated with bonding is the contamination of the cut surface by saliva and other oral fluids. Since inorganic components of the tooth structure have a greater affinity for these flids, adequate isolation is essential for optimal bonding.
• Water: Dentinal tubules consist of water which will compete with the adhesive liquid to flow and fill the crevices. Etched enamel and dentin have a greater affinity for water; this poses a problem when hydrophobic bonding agents are used prior to composite resin restorations.

2. Cohesion:

Cohesion is the force of molecular attraction between molecules or atoms of the same species, for example, Water molecules. As per the Glossary of Prosthodontic Terminology (GPT) cohesion is the molecular attraction by which the particles of the body are united throughout their mass.

This is based on the principles of dipole attraction due to the uneven distribution of electrons around the atoms or the molecules. It happens when two, unlike molecules or substances, join together on being brought in contact because of the force of attraction between them, for example, the Cohesion of salivary molecules interposed between denture and tissue surface.

Application of Cohesion in Dentistry: Pure gold either in foil or other form is cohesive and can be welded to itself at the mouth or room temperature simply by the application of force.